1. Field of the Invention
The present invention relates to an active suspension system for an automotive vehicle. More specifically, the invention relates to an active suspension system which is provided high sensitivity in detecting vehicular attitude change, such as rolling and pitching, for providing high level stability of the vehicular attitude.
2. Description of the Background Art
U.S. Pat. No. 4,865,347, discloses an actively controlled suspension system including an anti-roll control loop which is responsive to lateral acceleration to be exerted on a vehicle, to adjust suspension characteristics. The control loop employed suspension system is variable of responsive characteristics to vehicular rolling depending upon the vehicle speed for varying anti-rolling suspension control characteristics. In a range of the vehicle speed in which the greater loop gain of the control loop causes significant and unacceptable level of self-induced lateral vibration, the gain is varied to be of smaller value than that in other vehicle speed range. Since the greater gain may cause self-induced lateral vibration in substantially low vehicle speed range, the gain main be adjusted to be smaller while the vehicle speed is lower than a given value.
U.S. Pat. No. 488,696, discloses an actively controlled automotive suspension system employing a factor representative of self- induced attitude change as a correction factor in attitude regulating suspension control. Suspension characteristics are controlled on the basis of a rolling and/or pitching energy representing parameter. The correction factor representative of self-induced attitude change is derived on the basis of vehicular attitude change velocity for canceling the self-induced rolling and/or pitching. By canceling the self induced rolling and/or pitching creative component from the rolling and/or pitching acceleration factor, higher rolling and/or pitching response characteristics can be provided so as to satisfactory high driving stability and riding comfort.
U.S. Pat. No. 4,872,701, discloses a suspension control system for an automotive vehicle which is designed for effectively suppressing pitching motion and whereby regulating vehicular attitude. The suspension control system monitors a pitching moment to be exerted on the vehicle for adjusting response characteristics in pitching-suppressive control depending upon the magnitude of the pitching moment to be exerted. The suspension control system adjustments the response characteristics of the front and rear suspensions independently of each other according to preset pitching-suppressive control coefficients respectively for the front and rear suspensions, which are set depending upon the suspension characteristics and/or suspension geometry of the vehicle, to which the suspension control system is to be applied.
In the type of the active suspension systems which performs attitude change suppressive control on the basis of longitudinal and/or lateral acceleration exerted on the vehicular body, difficulty is encountered by a performing attitude change suppression when vehicular attitude change, e.g. rolling or pitching, is induced in cause other than longitudinal or lateral acceleration. Such type or vehicular rolling or pitching can be induced due to undulation on the road surface when the road surface condition is not smooth.